I recently picked up an HP oscilloscope from a local university. I was playing around with it last night with no troubles, but after firing it up this morning it seems channel one is having some issues.

In the image, I'm testing the calibration output on both channels, but only number one seems to be spiking up before slowly dropping off. This persists even after trying several different probes, so I know its a fault inside the machine.

I'm not an expert, but if I were to pop it open, my hunch would be a faulty capacitor somewhere close to the channel input that needs replacing, but I thought id post my problem online first; see if someone with a little more experience knows whats going on.

I'm guessing that's only because of the oscilloscope probe ground clip. If you shorten the ground lead on the probe, the spike will probably decrease in height. This is normal, and is exactly why you have to be careful what probes and clip styles you use for high-precision circuits and measurements.

Matt

P.S. Would you mind reducing the size of the photo? It's blown way up and makes it a bit more difficult to see all at once Thanks!

I'm guessing that's only because of the oscilloscope probe ground clip. If you shorten the ground lead on the probe, the spike will probably decrease in height. This is normal, and is exactly why you have to be careful what probes and clip styles you use for high-precision circuits and measurements.

Matt

P.S. Would you mind reducing the size of the photo? It's blown way up and makes it a bit more difficult to see all at once Thanks!

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Whoops, sorry about that. Fixed.

Anyway, I'll have to try that when I get a chance. If that were the case though, why would it be exclusively on channel one, no matter which probe I use?

I see one probe and one (presumably) BNC-Alligator cable. If you're using the BNC-Alligator to connect to channel 1, then that's probably the issue. There is a relatively significant amount of inductance in those types of cables, and little filtering. Coupled with the scope's input capacitance, it's probably causing some oscillation.

This is the same thing that causes it to happen in some scope probes, just more significant.

Try using scope probes on both, and let me know what the waveform looks like.

Oh, I was just using the clip for the picture so I could hold my camera. I tested it out earlier with 2 different probes, and tried both the x1 and x10 settings on them with no change; flawless on channel 2, but underpowered and dropping off on channel 1.

Overshoot as shown, can also be caused by incorrect compensation on X10 probes. Such probes have a small adjustment screw that matches the probe capacitance to the input capacitance to give a normal looking square-wave.

The fact that both channels are measuring the same point, and have the same volts/div setting, yet the amplitude on channel 1 is about half that of channel 2 makes me wonder. Might be worth popping the thing open and looking for any bulging/blown caps, burnt resistors, etc. I've mentioned just about everything else I can think of, and you seem to have it under control.

If you're measuring the same point, you shouldn't have an unsteady image when you have the time/div set correctly....

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It shouldn't, but the signal through channel 1 is so weak and a bit spotty that it wobbles quite a bit when I use it for triggering.

Anyway, my thoughts exactly matt. On the one hand, I'm a little upset my new toy isn't working properly, but on the other, fixing it sounds like a lot of fun, and if it is just a cap, it shouldn't be too hard I hope. Thanks for helping me narrow it down a bit!

Overshoot as shown, can also be caused by incorrect compensation on X10 probes. Such probes have a small adjustment screw that matches the probe capacitance to the input capacitance to give a normal looking square-wave.

But in your case it sounds like a faulty channel.

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Probe compensation is a possibility - what happens trying each probe on each channel with the calibration signal?

Sadly, probe compensation isn't the issue. I can make minor adjustments with it, but the signal is still way weak and spotty.

Sadly, I didn't see anything obviously fried when I popped it open, so I'm thinking rather than mess with it myself and spend days testing all the components and risking wrecking something else, I'll just have to suck it up and give it to a friend of mine who isn't a complete amateur like I am. Not as satisfying for sure, but ah well.

Sadly, probe compensation isn't the issue. I can make minor adjustments with it, but the signal is still way weak and spotty.

Sadly, I didn't see anything obviously fried when I popped it open, so I'm thinking rather than mess with it myself and spend days testing all the components and risking wrecking something else, I'll just have to suck it up and give it to a friend of mine who isn't a complete amateur like I am. Not as satisfying for sure, but ah well.

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There's still a lot of people upgrading to LCD scopes, and very nearly giving away their old CROs - make the best of it for now and keep watching for another bargain.

Instrument servicing/calibration isn't cheap - if by chance you stumble on another the same model, you have some spares - otherwise some interesting parts to harvest.